Structures, such as removable bridges, for crossing difficulties in terrain are well known. Such difficulties may include, for example, gaps formed by ditches, canals and rivers.
Although the construction of a bridge for light traffic is comparatively straightforward, providing a bridge capable of supporting heavy ground equipment is of considerably greater difficulty.
In one approach, a relatively lightweight launching rail or beam is first extended across the gap to be spanned. This rail acts as a load carrying member, to support a modular bridge. The bridge is suspended from the rail and then extended across the gap. In that case, the overhead rail has the advantage that, by using a wire rope and winch system, power can readily be provided to propel the bridge across the gap and subsequently to lower it onto the banks.
The structure described above is particularly suitable for undecked twin girder bridges, that is, bridges having generally parallel, spaced girders in a longitudinal direction but without transverse deck members. The rail is deployed from a central pylon between the two girders. Such an arrangement is advantageous because the pylon can be easily designed to react to the very large cantilevered loads associated with launching the rail.
Difficulties exist, however, in applying the above arrangement to pre-decked bridges, that is, bridges containing transverse deck members which are affixed to the longitudinal girders prior to extending the bridge across a gap. In particular, it is difficult to design a structure from which the rail can be launched and which does not obstruct the space beneath the rail.
One possible solution would be to provide a low level rail over which the bridge is launched. However, this has significant disadvantages in that the rail must be left in place after the bridge is built. This approach also leads to unattractive complexity in providing power for the movement of the bridge in both the horizontal and vertical planes.